Agriculture Reference
In-Depth Information
barberry eradication in the early 20th century (which enabled the survival of
P. graminis
f.sp.
tritici
in the aecidial stage) and from the infrequent occurrence
of favourable temperatures. Stem rust occurs in North America, Australia and
Argentina, but because of the use of resistant cultivars, its incidence is usually very
low nowadays. Selection for earliness in European cultivars contributed to their
escape from stem rust. The epidemiology of stripe rust has been reviewed by Rapilly
(1979), Stubbs (1985), and Line (2002), that of leaf rust by Samborski (1985) and
Eversmeyer and Kramer (2000), and stem rust by Roelfs (1985a,b) and Eversmeyer
and Kramer (2000), and the genes for resistance to rust fungi in wheat by McIntosh
et al.
(1995b).
The aim of this section is to compare cereal powdery mildews and rusts which
have features in common, such as wide distribution, rapid development within or on
host tissue, massive production of spores, the ability to remain viable after long-
distance dispersal and a high capacity to become virulent on previously resistant
cultivars. The first part compares monocyclic components of the fungal infection
cycle, including spore dispersal, influenced by meteorological and biotic factors
(e.g. de Vallavieille-Pope
et al.,
2000). This knowledge forms the basis for the
development of models describing epidemics of mildews (e.g. Hau, 1985) and of
rusts (e.g. Shrum, 1975). The chapter will then continue with survival strategies in
connection with the sexual state and, finally, with the dynamics of sub-populations
defined by virulences or fungicide resistance.
15.2 METEOROLOGICAL AND BIOTIC EFFECTS ON THE PHASES OF THE
ASEXUAL LIFE CYCLE
Mildews and rusts are polycyclic diseases, which can complete the asexual cycle
several times within a season. For mildews, the asexual cycle is the production of
haploid conidia while the ascospores, which are the result of the sexual cycle, can
occasionally initiate epidemics. For rust fungi, the life cycles are more complicated
because macrocyclic heteroecious rusts can develop five types of spores:
urediniospores (binucleate), teliospores (in which meiosis occurs), basidiospores
(uninucleate and haploid) on the cereal host, and pycniospores (uninucleate, with
plasmogamy between different mating types) and aeciospores (binucleate) on the
alternate host. The exception is
P. striiformis
f.sp.
tritici
, for which the alternate host
is unknown. Rust epidemics on cereals are mainly caused by the dikaryotic
urediniospores.
For cereal mildews, it is generally assumed that the different
formae speciales
react identically to environmental conditions so that powdery mildews on wheat and
on barley are driven by the same factors. The different rust species on wheat,
however, have different environmental requirements.
15.2.1 Infection
The infection process of powdery mildew and rust fungi can be subdivided into
germination, elongation of the germ tube, appressorium formation, penetration and
